1. Field of the Invention
The present invention relates generally to thermal ink-jet printing, more particularly to free-ink ink-jet pens and, more specifically to a dual function thermal control mechanism for ink-jet print heads.
2. Description of Related Art
The art of ink-jet technology is relatively well developed. Commercial products such as computer printers, graphics plotters, copiers, and facsimile machines employ ink-jet technology for producing hard copy. The basics of this technology are disclosed, for example, in various articles in the Hewlett-Packard Journal, Vol. 36, No. 5 (May 1985), Vol. 39, No. 4 (August 1988), Vol. 39, No. 5 (October 1988), Vol. 43, No. 4 (August 1992), Vol. 43, No. 6 (December 1992) and Vol. 45, No. 1 (February 1994) editions. Ink-jet devices are also described by W. J. Lloyd and H. T. Taub in Output Hardcopy Devices, chapter 13 (Ed. R. C. Durbeck and S. Sherr, Academic Press, San Diego, 1988).
In the art, it is known to provide a print head having an orifice plate that operates in combination with subjacent heating elements, such as resistors. Thermal excitation of ink is used to eject droplets through tiny nozzles in the orifice plate onto an adjacent print medium. The combination of a nozzle with an orifice, an ink manifold, and a firing resistor is sometimes referred to simply as a "drop generator" or an "ejector." Generally, the print head is scanned across the print medium and dot matrix manipulation is performed to create a graphics or photographic images or alphanumeric characters from patterns of individual ink droplets at particular locations that can be described as a linear matrix array of picture elements ("pixels").
The ink-jet print head mechanism itself may have a self-contained reservoir (referred to in the art as "on-axis") for storing ink and providing appropriate amounts of ink to the print head during a printing cycle. These self-contained, disposable mechanisms are often referred to as "pint cartridges."
If a refillable type "pen" rather than a print cartridge is employed in the hard copy apparatus, ink is generally supplied from a remote, refillable or replaceable, offboard ("off-axis")ink reservoir which is coupled by an ink conduit to a relatively permanent pen body and print head mechanism. Alternatively, such a "free-ink" ink-jet printing mechanisms have also been designed to have a print head mechanism and a detachable, on-board, reservoir that can be refilled or replaced as needed. The ink-jet pen and particularly the print head element is thus expected to have a longer life than a disposable cartridge.
Early in the development of thermal ink-jet printing it was discovered that the preheating of ink in the vicinity of the ink drop firing resistors has many advantages, as explained for example in U.S. Pat. No. 4,490,728 (Vaught et al., 1984, assigned to the common assignee of the present invention and incorporated herein by reference). The electrical pulse to each resistor comprises a "precursor pulse" and a "nucleation pulse." The precursor pulse preheats the ink in the vicinity of the resistor to a temperature below the boiling temperature of the ink so as to preheat the ink while avoiding vapor bubble nucleation within the local ink supply. Subsequently occurring nucleation pulses very quickly heat the resistor to near the superheat limit of the ink, causing an ink droplet to be ejected through the nozzle. Thus, temperature sensing, or monitoring, of the print head mechanism also became an important operational parameter.
Various means have been invented to accomplish a preheating function in thermal ink-jet print heads. See e.g., U.S. Pat. Nos. 4,704,620; 4,899,180; 4,910,528 (Firl et al., assigned to the common assignee of the present invention); 5,107,276; and, also assigned to the common assignee of the present invention: 5,109,234; 5,144,336; 5,168,284; 5,235,346; 5,418,558 (Firl et al.); 5,428,376; and 5,475,405. Each of these techniques has its advantages and disadvantages.
It has been found, however, that there is a need for a mechanism allowing a preheating of the print head in a solid state fabrication ink-jet print head such that the prior art's complicated and chip area consuming logic are no longer required to accomplish the preheating function.